Author

Abstract

A reference section for the Albian to Lower Cenomanian interval is composited from outcrops exposed in the Trinity River valley in the area of Fort Worth, north-central Texas. The composite section represents the Gulf Coast Comanchean Series that consists of three groups, Trinity, Fredericksburg, and Washita. These groups represent second-order transgressive- regressive depositional cycles. The Washita is divided into six third-order cycles of flooding shale and marl containing basal lenticular channels of storm deposits and that grade up to carbonate shelf wackestones.

Biostratigraphy of the Comanchean Series is based upon ammonites, foraminifers, nannofossils, palynomorphs, and selected bivalves. The middle/upper Albian boundary is identified by the Dipoloceras cristatum Zone and nannofossils in well-exposed sections of marl and limestone representing a complete depositional cycle. The upper Albian/lower Cenomanian boundary is identified in the uppermost meter of the Main Street Limestone at a cycle boundary, which correlates by graphic correlation with the proposed boundary in France. The first occurrence of the nannofossil, Microrhabdulus decoratus, is directly above. This section may be useful as a supplemental section to the potential boundary stratotype.

The upper Albian-lower Cenomanian depositional cycles in the Washita Group are defined accurately in the composite section and integrated with diagnostic bioevents. Thus, they can be projected into other sections by techniques of quantitative stratigraphy, such as graphic correlation. This allows the precise correlation of these cycles with cycles in other parts of the Comanche Shelf and in other basins, thereby enabling the testing of exact synchroneity between cycles. In addition, precise ages of strata at cycle boundaries, of cycle durations, and of rates of sediment accumulation can be interpolated by graphic correlation and tested by future data.